Trim the rhubarb and cut into 3cm chunks. Place the rhubarb and 1 tbsp of the sweetener in a nonstick saucepan. Add the water, cover the pan with a lid and cook over a low heat for 10 minutes or until the rhubarb is very tender. Remove from the heat and leave to cool.
Stir the rhubarb to break up the chunks. Add 3-4 tbsp more sweetener to taste and stir well. Using electric beaters, whisk the double cream with 1 tbsp of the sweetener and the vanilla extract in a large bowl until soft peaks form. Gently fold in the Crème fraîche then the rhubarb.
Spoon into six glass tumblers or wine glasses. Cover with cling film and chill for at least 2 hours. Remove the cling film and crumble a biscuit onto each fool. Decorate with fresh mint, if you like, and serve immediately.
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